Polymeric membrane based separation processes such as reverse osmosis, pervaporation and perstraction are conventional. In the pervaporation process, a desired feed component, e.g., an aromatic component, of a mixed liquid feed is preferentially absorbed by the membrane. The membrane is exposed at one side to a stream comprised of a mixture of liquid feeds and a vacuum is applied to the membrane at the opposite side so that the adsorbed component migrates through the membrane and is removed as a vapor from the opposite side of the membrane via a solution-diffusion mechanism. A concentration gradient driving force is therefore established to selectively pass the desired components through the membrane from its upstream side to its downstream side.
The perstraction process is utilized to separate a liquid stream into separate products. In this process, the driving mechanism for the separation of the stream into separate products is provided by a concentration gradient exerted across the membrane. Certain components of the fluid will preferentially migrate across the membrane because of the physical and compositional properties of both the membrane and the process fluid, and will be collected on the other side of the membrane as a permeate. Other components of the process fluid will not preferentially migrate across the membrane and will be swept away from the membrane area as a retentate stream. Due to the pressure mechanism of the perstraction separation, it is not necessary that the permeate be extracted in the vapor phase. Therefore, no vacuum is required on the downstream (permeate) side of the membrane and permeate emerges from the downstream side of the membrane in the liquid phase. Typically, permeate is carried away from the membrane via a swept liquid.
The economic basis for performing such separations is that the two products achieved through this separation process (i.e., retentate and permeate) have a refined value greater than the value of the unseparated feedstream. Membrane technology based separations can provide a cost effective processing alternative for performing the product separation of such feedstreams. Conventional separation processes such as distillation and solvent extraction can be costly to install and operate in comparison with membrane process alternatives. These conventional based processes can require a significant amount of engineering, hardware and construction costs to install and also may require high levels of operational and maintenance personnel costs to maintain the associated facilities in an operating status. Additionally, most of these processes require the heating of the process streams to relatively high temperatures in order to separate different components during the processing steps resulting in higher energy costs than are generally required by low-energy membrane separation processes.
In general, the membrane technology in the present art has the benefit of lower per unit energy costs per volume of separation than the conventional technologies in present art. However, a major obstacle in perfecting the commercial operation of membrane separation technologies is to improve the flux and selectivity characteristics of the membrane systems in order to make the construction costs and separation efficiencies of membrane technologies economically viable, for example, on a refinery scale operations and on-board vehicle separation processes.
A myriad of polymeric membrane compositions have been developed over the years. Such compositions include polyurea/urethane membranes (U.S. Pat. No. 4,914,064); polyurethane imide membranes (U.S. Pat. No. 4,929,358); polyester imide copolymer membranes (U.S. Pat. No. 4,946,594); polyimide aliphatic polyester copolymer membranes (U.S. Pat. No. 4,990,275); and diepoxyoctane crosslinked/esterfied polyimide/polyadipate copolymer (diepoxyoctane PEI) membranes (U.S. Pat. No. 5,550,199).
Major factors affecting the performance (i.e., the selectivity and flux rate) of a polymeric membrane are the composition of the membrane material, the concentration of the membrane material in solution, the curing or chemical reaction methods, and the final thickness of the cast membrane. In general, for a polymeric given membrane composition, the flux across a given membrane is approximately inversely proportional to the thickness of the membrane. Therefore, the active portion of membranes in the prior art are generally cast as very thin films (on the order of 0.1 to 50 micron thickness) in order derive the selectivity benefit of the membrane while maximizing the flux characteristics of the membrane. However, problems associated with the fabrication and operation of thin membranes include voids and inconsistencies in the membrane structure which affect the membrane performance as well as mechanical instability of the membrane due to their thin structural profile.
Copolymeric membranes of the prior art may be comprised of “soft segments” and “hard segments” and may undergo a “thermal cross-linking” at relatively high temperatures (above approximately 300° C./572° F.) to provide a inter-chain structural framework to impart mechanical and thermal strength to the membrane. While the soft segments of the polymer provide the active area for the selective diffusion of the permeate through the membrane, they generally possess limited structural and thermal strength characteristics. Therefore, in order to provide the membrane sufficient structural integrity, the polymer soft segments are polymerized with the hard polymer segments to form copolymer chains. In this way, the hard segments of the copolymer chain provide the necessary mechanical and thermal strength to the overall membrane. However, these hard segments of the copolymer chains possess limited, if any, permeability of the process stream components. The problem that exists is that the copolymer membranes that result from thermal cross-linking, while relatively mechanically stable, have limited flux and/or selectivity characteristics.
A specific problem that exists in the industry is that many of the membranes in the art have a hard or “glassy” composition texture. These membrane compositions do not allow for subsequent layers to be applied and be integrally bonded with the preceding membrane layer. These single-layered membranes have limited dimensionalities of separation properties without the improved and/or tailored separation performance characteristics that may be attained through the synergistic effect of integrating multiple differing copolymer concentrations or compositions into a single membrane.
Therefore, there exists in the industry a need for a selective membrane which compositional characteristics can be tailored by integrating multiple membrane layers of varying polymer concentrations or compositions resulting in a multi-layered membrane with improved separations performance characteristics.